I am attempting to get IRRs using the XIRR function. This is being used in
a model in which the dates are dynamic and can be changed. The XIRR function
is being used over a number of time periods, i.e. a 1-yr, 2-yr ... up to 8-yr
time period.

Some of the dates that I have chosen are giving me a #NUM error.
For example if I start the date sequence with July, 2008 in the 1-yr time
sequence, and include as other dates in that sequence Aug, 2008, etc I get
the #NUM error, but if I start that sequence with July 2008 in order to not
get the error I have to include a date that is many years out for example

August 1, 2008
August 1, 2010
December 1, 2013
January 1, 2014
February 1, 2014
March 1, 2014

The above scenario gets me an acceptable result (which is unrealistic for
the model) but the following which is more realistic gets me a #NUM error

August 1, 2008
August 1, 2008
December 1, 2008
January 1, 2009
February 1, 2009